This application relates to control of contaminating particles in materials processing.
In a sputter or reactive ion etching or chemical vapor deposition apparatus used in semiconductor manufacturing, a semiconductor wafer is placed between two electrodes which are driven by a radio frequency (RF) AC voltage source. (Ordinarily the wafer is placed directly over one electrode and a substantial distance from the other electrode.) A reactant gas is released into the chamber in the area above the wafer. Because the reactant molecules are relatively large and immobile, they cannot move in response to the rapidly fluctuating electric fields. However, the electrons associated with the reactant molecules are relatively mobile. As a result, if the AC electric fields are sufficiently intense, electrons are stripped from the reactant molecules, forming a gas plasma of free electrons and positive reactant ions. This plasma is highly conductive because it contains a large number of free carriers, and therefore is essentially an equipotential region.
Ions have a much larger mass than electrons; in fact, ions are so massive that they are essentially unaffected by the RF AC electric fields; however, due to their smaller mass, free electrons are rapidly accelerated by the RF AC electric fields. Note, however, that because the plasma is an equipotential region, electrons within the plasma are not influenced by the RF AC electric fields; only those which reach the edge of the region between the plasma and the wafer--the "plasma sheath"--are accelerated.
When the RF AC source is first turned on, the wafer and electrode surfaces contain no charge. In the first half of the AC cycle the electrode immediately underneath the wafer has a positive voltage, and the wafer surface attracts electrons and repels ions. During this half of the cycle, electrons are accelerated from the sheath onto the wafer, where they join other electrons in one of the energy bands of the wafer's semiconductor material. In the second half of the AC cycle, the electrode underneath the wafer has a negative voltage, which attracts positive ions toward the wafer and repels electrons. However, relatively few of the electrons on the wafer develop the necessary energy to leave the energy bands of the wafer and return to a free energy state within the plasma. Thus, during the first and subsequent AC cycles, electrons are collected on the wafer surface.
This accumulation of negative charge causes the wafer to "self bias" to a negative DC voltage relative to the plasma, creating a DC electric field in the sheath. Although the ions in the sheath are unaffected by the AC fields, they are accelerated toward the wafer by these DC fields, resulting in a DC flow of ions onto the wafer surface. At the same time, as the self bias voltage builds, during a larger part of the AC cycle the wafer voltage is more negative than the plasma, reducing the time during which electrons are attracted to the wafer. Ultimately, the wafer charges to a negative voltage nearly equal to the peak voltage of the AC signal, and the system reaches a steady state in which an equal number of positive ions and electrons strike the wafer during each AC signal period. The flow of ions is relatively steady throughout the cycle, and is equalized by a brief spike of electron flow during the peak portion of the AC cycle.
Particle contamination is becoming an increasingly serious limitation in high quality materials processing such as semiconductor manufacturing. In the semiconductor manufacturing area, it has been estimated that as much as 70-80% of all wafer contamination is caused by particles. Thus, substantial reductions in defect rates may be achieved by reductions in particle contamination.
A typical plasma apparatus includes many potential sources of contaminating particles, such as: cracked or flaking materials (e.g., quartz) or films (e.g., dielectric films) inside the chamber, polymers collecting on the walls of the chamber over time, small metal spheres created by arcing between metal surfaces, and incidental contact or rubbing inside the chamber during wafer handling. Once particles from any of these sources are released into the processing chamber during processing, they enter the gas plasma, and ultimately land on and contaminate the wafer surface.
Because free electrons within the plasma have a much higher mobility than positive ions, particles entering the plasma are bombarded with more electrons than ions, and build up a negative surface charge. Gravitational forces pull the particles down in the direction of the wafer, but once the particles reach the plasma sheath, the strong DC electric field between the plasma and the negatively charged wafer overcomes the downward gravitational force, and as a result the particles float at the plasma sheath boundary. However, when the RF power is turned off, the negative charge on the wafer dissipates, and any particles at the sheath boundary fall onto and contaminate the wafer 10.
Selwyn, "Particle trapping phenomena in radio frequency plasmas" (Applied Physics Letter 57, Oct. 29, 1990) photographed particles floating above wafers using scattered laser light. As illustrated in FIGS. 1A and 1B, Selwyn discovered that when the wafer 10 is inserted into a recess of a graphite electrode 11, levitating particles tend to trap above the wafer, forming a cloud over the wafer in the shape of a ring 12 approximately above the outer edge of the wafer.
Selwyn, "Plasma particulate contamination control II. Self-cleaning tool design" (Journal of Vacuum Science and Technology, 10(4), July/August, 1992) discusses this effect and shows that the trapping phenomenon can be reduced by grooving electrode 11. The groove creates a channel in the sheath leading away from outer ring 12 and toward the vacuum inlet port. Particles that would have been trapped in the outer ring 12 follow this channel, thus reducing the number of particles in the area above the wafer.